Elusive Common

difficult to describe

"a haunting elusive odor"

skillful at eluding capture

"a cabal of conspirators, each more elusive than the archterrorist"

difficult to detect or grasp by the mind or analyze

"his whole attitude had undergone a subtle change"

"a subtle difference"

"that elusive thing the soul"

making great mental demands; hard to comprehend or solve or believe

"a baffling problem"

"I faced the knotty problem of what to have for breakfast"

"a problematic situation at home"

Evading capture, comprehension or remembrance.

"Despite hours of searching through old photographs, my grandmother's face remained elusive in that faded album."